Added a space between the class name and the event type in AXEvent's ToString method

This enhances readability when debugging.

AX-Relnotes: n/a.

TBR=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: I4e0bcf1ccb7ded6b0204b9c06b9965bde76b05f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249934Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779782}

ui/accessibility/ax_event.cc

@@ -29,7 +29,7 @@ AXEvent::AXEvent(const AXEvent& event) = default;
 AXEvent& AXEvent::operator=(const AXEvent& event) = default;
 
 std::string AXEvent::ToString() const {
-  std::string result = "AXEvent";
+  std::string result = "AXEvent ";
 
   result += ui::ToString(event_type);
   result += " on node id=" + base::NumberToString(id);